<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    function isLeapYear(year) {
      var flag = '平年'  // 假设不是闰年

      if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
        flag = '闰年'  // 是闰年
      }

      return flag
    }

    function februaryDay() {
      var year = parseInt(prompt('请输入年份'))

      if( isLeapYear(year) === '平年') {
        alert('28')
      } else {
        alert('29')
      }
    }

    februaryDay()

  </script>
</body>

</html>